Miroku
Apart from the consumables, most of the other stuff he is selling isn't quite needed or useful to you and are pretty costly to purchase. You should especially avoid using his "Bargain" option to buy anything other than friendship increasing presents to give to your NPC Partners (and perhaps some rare or otherwise hard to obtain stuff like specific crafting ingredients).
(* Note that the variety of items, quantity in stock and stuff he offers in "Bargain", all of them depends upon your Friendship Level with him.
- Assortment (品揃え) - Affects the variety of items he sells in his store.
- Stock (在庫) - Affects the quantity of the items he sells in his store.
- Negotiation (商談) - Affects the kinds of stuff he puts up for Bargain Sales.